The present invention relates to coalesced rubber particles having an average particle size in the range of 5 microns up to and including 50 microns, and to epoxy resin blends containing these coalesced rubber particles, also known as microparticles.
There has been continued growing interest in fiber-reinforced prepreg materials for use in aerospace, automotive, sports and other high performance applications. The continuous fibers embedded in a polymer matrix provide load bearing strength to such composite materials. Epoxy resins are thermosetting matrices commonly used in prepreg materials because epoxy resins are characterized by dimensional stability, mechanical strength, electrical insulating properties, heat resistance, water resistance, and chemical resistance. However, the cured epoxy matrix in a prepreg has poor fracture toughness and is very brittle. One method of increasing the toughness of an epoxy matrix is to add a thermoplastic material or a rubber. A common toughening method is to disperse rubber modifiers through the entire epoxy matrix in a homogeneous manner.
Multilayer fiber-reinforced prepreg materials have been developed which are laminates of layers of fibers and resin. In such layered composites it is desirable for the particles of rubber modifier to be concentrated in the layers of resin separating the layers of fiber (or fiber tows) as opposed to being embedded inside the fibers, to achieve effective toughening of the resin layers and prevent delamination of the composite.
A common method for making a prepreg ply having preformed rubber particles concentrated on the surfaces of a layer of fibers is by single pass impregnation. In this method two films of particle-modified resin are formed, for example on a release sheet, and respectively applied to each side or surface of a layer of fibers or fiber tow. This ply is then subjected to a hot melt treatment with the application of pressure to impregnate the fiber tow with the resin. With the selection of appropriate conditions and materials, the rubber particles filter out through the fibers during the prepreg process and concentrate on the surfaces of the ply. This method has the advantage of being relatively simple, but may lead to problems in prepreg quality related to the distribution of the particles. The size of the rubber particles is a particularly important factor in single pass impregnation. The use of modifier particles from 1 micron to 100 microns in size has been reported for toughening the resin matrix in a prepreg, and in particular for toughening the resin interlayer between fiber tows. However, problems in prepreg quality have been reported when the modifier particles are at the lower end of this range of particle size. In particular, as the particle size is decreased and approaches the diameter of the reinforcing fibers, the particles may fail to filter out during the single pass impregnation, and instead impregnate the fiber bed. In such case, the particles are dispersed throughout the composite and the interlayer toughening effect is not achieved. As the particle size of preformed rubber particles increases in an epoxy-based multilayer prepreg, more unevenness is observed in the rubber-modified resin interlayer. The fracture toughness in this type of composite is also affected by the particle size, as well as the particle size distribution and the overall concentration of particles. (Hayes et al., Journal of Composite Materials 36: 299 (2002))
The present invention provides coalesced (preformed) rubber particles having an average particle size in the range of 5 microns up to and including 50 microns, which are particularly suitable for use as modifiers for epoxy resins in prepreg materials, including multi-layer prepregs.
Crosslinked rubber particles of about 50 microns in size are currently produced by a spray drying method. However, rubber particles of about 10-20 microns in size cannot be made readily by the spray drying method because of the risk of ignition associated with spray drying very small particles. EP 1632533 A1 discloses a process for producing an epoxy resin composition having core/shell rubber particles (0.05 to 1 micron in size) dispersed in the epoxy resin. This particle size is too small for use in multi-layer prepregs. The disclosed method could not produce particles of a selected size in the range of 5-40 microns. Other master-batch blends of modified epoxy have been proposed. JP 5-295237 A discloses a method in which a rubber-like polymer latex is coagulated, the coagulated material is dried until the moisture content is less than 15 wt % and then pulverized. The resulting powder is mixed with an epoxy resin. This method suffers from a number of drawbacks: the coagulated material tends to form a rubber sheet after the milling step because of the low glass transition temperature of the rubber; it is very difficult to obtain the desired particle size; and the residual water could not be removed readily because of the high boiling point of water. JP 6-107910 A discloses a method in which a rubber-like polymer latex is mixed with an epoxy resin, and then the water is distilled away to give a modified epoxy blend. A difficulty with this method is that a large amount of water must be removed in the presence of the epoxy resin at the risk of causing opening of the epoxy ring. It is also very difficult to obtain the desired particle size by this method. U.S. Pat. No. 4,778,851 discloses a number of methods for preparing rubber-modified epoxy resins. In one of the disclosed methods a core-shell rubber latex is mixed with an epoxy resin in the presence of an organic solvent. A major drawback of this method is that a large amount of water present in the mixture and the organic solvent must be removed, for example by distillation. The separation of the aqueous layer from the organic solvent layer takes a long time, which can be as much as a whole day, and may be hampered when the organic solvent layer and the aqueous layer form a stable emulsified or suspended state.